The present invention relates generally to communication line testing systems and, more specifically, relates to a testing system and method which provides for selective connection of testing equipment to any of a plurality of high speed digital communication lines or any other very high frequency digital transmission lines.
The term T-1 refers to a telecommunications standard for digital transmission used extensively in the United States. The T-1 standard provides a transmission link with a capacity of 1.544 megabits per second (Mbps) over a twisted wire pair. With this capacity, a T-1 link can handle the equivalent of 24 voice conversations, each digitized at 64 kilobits per second (Kbps). However, with the ever increasing demands that modern technology and the information super highway places upon the communications industry, increasing bandwidth is being demanded. In response to such demand, faster communication links, such as T-3 transmission links, are being deployed to meet these demands. A conventional T-3 link provides the equivalent of 28 T-1 links or a capacity of 44.736 Mbps, which is the equivalent of 672 voice conversations. A T-3 line typically runs on fiber optic, microwave radio, or coaxial cable lines.
The signaling protocol for T-3 systems, commonly referred to as DS-3 signaling, involves pulses which require a bandwidth comparable to VHF (very high frequency) radio waves. At these frequencies, providing switchable access between communication links and test equipment can become problematic, because of the need to ensure signal integrity as the DS-3 pulses propagate through the system. For example, at the circuit level, solid state switching devices are no longer effective to make switchable connections, owing to the high frequency parasitic circuit paths present in such devices. On the printed circuit board level, it becomes necessary for circuit paths to appear substantially as transmission lines, and any failure to do so, can result in substantial mismatches, reflections and other signal distortions, in addition to crosstalk, on the circuit board itself.
Broadly, it is an object of the present invention to provide a method and apparatus for switching a plurality of testing devices among a plurality of transmission links while preserving the integrity of the signal as it propagates through the system. It is specifically contemplated that all signal paths in the system exhibit the characteristics of a transmission line that provides for no appreciable attenuation or distortion of the signal and no appreciable crosstalk.
In accordance with a preferred embodiment demonstrating objects, features and advantages of the present invention, there is provided a system for providing selective testing access to a plurality of communication signal lines by a plurality of testing devices. The system includes line access cards that provide an interface for plurality of high frequency signal lines and at least one test card that provides an interface for a plurality of high frequency testing devices. The cards plug into a mother board, which provides selective connection between test devices and signal lines.
All high frequency signal paths on the cards and motherboard exhibit the characteristics of a transmission line with a predefined characteristic impedance, and transfer high frequency pulses with minimum attenuation, minimum distortion, and minimum crosstalk. Switching is provided on the motherboard by relays with low insertion loss and crosstalk. The relays are provided on the line access cards, test card, and motherboard. All signal paths represent straight point-to-point electrical circuits with no taps. The connections between the rear cards of the line access devices and the mother board are provided by 96 pin DIN connectors, and represent the only part of the high frequency signal path in which impedance is not strictly controlled. However, the signal integrity through these connectors is maintained by implementing a connector pin assignment and configuration which simulates a co-axial transmission line.